This section provides background information related to the present disclosure which is not necessarily prior art. It further provides a general summary of the disclosure, but not a comprehensive disclosure of its full scope or all of its features.
Generally, ice crystals and volcanic ash in large concentrations are hazards to aviation as they can lead to damage, power loss, and flameout in jet engines.
In fact, ice crystals icing has caused hundreds of jet engine power loss events in the past 30 years (e.g., Mason, 2007). Most of these events occurred in conditions that appear benign to an alert pilot. Moreover, they occurred well above the altitudes where the supercooled water droplets that cause the more common aircraft airframe icing exist. Power loss is defined as engine instability, such as a surge, stall, or flameout (engine shutdown), that results in extremely low power values. Small ice crystals in large concentrations cause these hazard events.
Volcanic ash can cause power loss and even damage jet engines while an airplane is flying in conditions that also appear benign to an alert pilot, in particular during night flights. Volcanic ash has caused a significant number of airplane incidents and air traffic disruptions in the past 30 years (e.g., Guffanti et al., 2010). This includes the shutdown of all four engines of a Boeing 747 without the pilots knowing its cause because of lack of awareness that they were flying in volcanic ash. The eruption of Iceland's Eyjafjallajökull Volcano in 2010 produced an ash plume that caused a weeklong closure of the airspace above Europe and the North Atlantic. The detection of volcanic ash could help pilots mitigate these problems.
Aircraft on-board weather radars are capable of detecting large particles, such as hail, rain, and snow, but not volcanic ash or the small ice crystals found in high concentrations near thunderstorms. Unfortunately, volcanic ash and small ice crystals are a major cause of jet engine power loss and flameout. Hazard ice crystals icing conditions can be inferred by detecting the presence of ice crystals in air recently lifted from the lower troposphere by thunderstorms updrafts. Hazard volcanic ash conditions can be inferred by detecting the presence of volcanic ash in air recently lifted from the lower troposphere by convective updrafts in volcanic plumes.
Radon-222 (radon) is a naturally occurring radioactive noble gas of terrestrial origin commonly used as atmospheric tracer (e.g., Zahorowski et al. 2004). Radon is produced by the decay of long-lived radium-226 present in rocks and soils. Radon is a good atmospheric tracer because by being a noble gas it does not react with any atmospheric constituent. Being inert and poorly soluble in water (Jacob and Prather 1990; Li and Chang 1996), radon is not removed by wet atmospheric removal processes such as rainfall. Indeed, the only significant atmospheric sink of radon is radioactive decay with a half-life time-scale of 3.82 days. Thus, vertical transport processes, such as convective updrafts in thunderstorms and in volcanic plumes, cause significant increases in radon activity in the upper atmosphere. In fact, measurements by NASA high-altitude research aircrafts demonstrate that radon can be used to identify air recently transported by thunderstorm updrafts even at altitudes above 15 km (e.g., Kritz et al., 1993).
Radon (radon-222) activity can be determined by measuring the radioactivity of radon and/or its decay products. Since radon has a half-life of 3.82 days, a particular radon nucleus may decay at any time but is most likely to decay in about 8 days (two half-lives). When a radon nucleus decays, it emits an alpha particle with 5.49 MeV of energy while the nucleus transforms into polonium-218. Polonium is a metal and therefore it sticks to things it contacts, such as aerosols, cloud particles and the surfaces of an instrument. Polonium-218 nuclei have a half-life of 3.05 minutes, which means that it is most likely to decay in about 6 minutes. Like radon, polonium-218 emits an alpha particle when it decays, but with energy of 6.00 MeV instead of radon's 5.49 MeV.
The detection of alpha radiation with solid-state detectors is frequently used for continuous measurements of radon activity. Solid-state alpha detectors are semiconductors (e.g. silicon) that convert alpha radiation directly into an electrical signal. This allows the energy of each alpha particle to be measured, making possible to determine which isotope (e.g., radon-222, polonium-218, polonium-214) produced it. Continuous measurement of radon requires instruments with low background noise, fast response, and fast recovery after exposure to high radon levels. Counting the alpha particles emitted by the decay of polonium-218 and ignoring the alpha particles emitted by the decay of polonium-214 that accumulates in the instrument, allows measurements with response of less than 10 minutes even for an instrument with sub-liter measurement chamber.
The concentrations of radionuclides, such as radon-222 (radon), are usually expressed in terms of disintegrations per unit time, defined as activity A=Nλ, where N is the number of radionuclide atoms, and λ is the atom radioactive decay constant. Common units of activity are disintegrations per minute (dpm), pico (10−12) Curie (pCi), and Becquerels (Bq); 1 Bq=60 dpm, and 1 pCi=2.2 dpm.
The flux of radon from the ground into the atmosphere is about 104 atoms/m2-s. Over the open oceans, the emission of radon is two to three orders of magnitude smaller than over land (Turekian et al. 1977; Lambert et al. 1982; Schery and Huang 2004), but radon activity over the open oceans is still orders of magnitude larger than kilometers above the surface (Kritz et al., 1993; Williams et al., 2011). Since the radon has a half-life of 3.82 days, it is not transported efficiently from the surface into the upper atmosphere by diffusion. Thus, radon activity in the upper troposphere and stratosphere is an excellent indicator of fast and recent transport by convective updrafts.
Radon activities in the upper troposphere and stratosphere, where jet aircrafts typically cruise are usually less than 0.5 pCi/scm (e.g., Moore at al., 1973), where scm is a standard cubic meter. In contrast, near the surface where convective updrafts originate radon activity is orders of magnitude larger than in the upper troposphere and in the stratosphere. Indeed, radon activity varies from more than about 5-10 pCi/scm over the oceans to more than 100 pCi/scm over land. Updrafts in thunderstorms and volcanic plumes transport radon rich near-surface air upwards because by being poorly soluble in water (e.g., clouds and precipitation) the radon concentration does not decrease significantly in storm updrafts and by being a noble gas radon does not react with atmospheric constituents.
Measurements by high-altitude aircrafts indicate that tropospheric air injected into the stratosphere by convective updrafts have mean radon activity of about 20 pCi/scm, an order of magnitude larger than the value observed in the upper troposphere (Kritz et al., 1993). Moreover, measurements inside and just above cirrus anvils indicate radon activity of the order of 10 pCi/scm, a significantly elevated value with respect to the background level of less than 0.5 pCi/scm at the same level (Kritz et al., 1993). Thus, elevated radon activity would be a good indicator that a cirrus cloud was formed in air recently originating in the lower atmosphere and therefore with large water content. Similarly large radon activity is expected in ash clouds because ash is lifted by updrafts originating at the surface where radon activity is high (of the order of 100 pCi/scm).
The aircraft hazards detection system of the present invention comprises a subsystem capable of detecting ice crystals and volcanic ash, as well as a radon activity sensor such as an alpha particle detector capable of identifying the presence of air recently lifted from the lower troposphere. The detection of ice crystals or volcanic ash in air recently lifted from the lower troposphere indicates a potentially hazards condition that can cause power loss and even damage jet engines.
Unfortunately, few systems exist that are capable of reliably detecting small ice crystals or volcanic ash, and providing an associated alert. Prior art approaches for detecting ice crystals include a variety of optical method. For example, U.S. Pat. No. 7,986,408B2 refers to a device for detecting and distinguishing airborne liquid water droplets from ice crystals by illuminating the target with a circularly polarized light beam and detecting changes in the light polarization. The backscattered light received by the device is converted into two linearly polarized components. These two components are then used to calculate parameters indicative of the presence or absence of airborne ice crystals and water droplets.
U.S. Pat. No. 6,819,265 refers to an ice detection system for monitoring the airspace ahead of the aircraft. The system comprises optical elements designed to direct pulsed laser beams into the airspace ahead of an aircraft. Another set of optical elements is used to receive the backscattered light and separate it into multiple wavelengths. Signals of the light detected at various pre-determined wavelengths are used to determine if airspace conditions ahead of the aircraft is likely to cause ice accretion on the surface of the aircraft.
U.S. Pat. No. 7,370,525 refers to a dual channel system for detecting ice accretion on aircraft surfaces. The system illuminates the surface of the aircraft with linearly polarized light. Optical systems with polarization sensitivity aligned to the transmitted light, and with polarization sensitivity orthogonal to it, are used to acquire the backscattered light. The ratio of the intensities of the light signals is used to determine the presence or absence of ice.
U.S. Pat. No. 6,269,320 refers to an in-situ Supercooled Large Droplet (SLD) detector. This system takes advantage of boundary layer flow patterns to detect SLD. It is capable of distinguishing between the presence of water droplets that cause regular cloud icing and SLD icing. This system detects ice after it accumulates on aircrafts surfaces, it does not provide warnings when a hazards situation occurs in the airspace ahead of an aircraft.
U.S. Pat. No. 6,091,335 refers to an optical device for detecting icing conditions outside the boundary layer of the aircraft. The device employs change in light polarization for detecting icing conditions. It includes a light source to illuminate an external measurement volume. Light backscattered by the water droplets in an external volume is measured using photodetectors. Analysis of the polarization of the light collected is used to estimate the severity of the icing conditions.
Prior art approaches for detecting volcanic ash also include methods for detecting changes in the polarization light emitted towards the target. For example, U.S. Pat. No. 9,041,926B2 refers to a method for determining the presence of volcanic ash within a cloud by analyzing changes in the polarization of light used to illuminate it. A circularly polarized light is used to illuminate the airspace ahead of an aircraft. Analysis of the backscattered light is used to determine the presence of volcanic ash within the cloud. This analysis includes the determination of the degree to which the polarization changes.
According to the principles of the present teachings, an ice crystals and volcanic ash detection system is provided that overcomes the disadvantages of the prior art and is particularly useful in detecting hazardous conditions invisible even to the most alert pilots. In most embodiments of the present teachings, the system detects ice crystals and volcanic ash recently lifted by measuring radon activity and making multi-spectral measurements of radiance.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.